Time Machine to Backup an external hard drive to a second external hard drive

Hi Everyone, I am trying to use Time Machine to backup an external hard drive to a second external hard drive. I click on Options at the bottom of the Time Machine window. This is where you can add or subtract items to "exclude these items from backups". In this section, the drive I would like to back up is on the list, but grayed out. Since it is grayed out, it is not allowing me to select the minus sign to remove it from the list of items to exclude from back up. Any ideas? I am on an iMac Pro with Big Sur. Thanks!